Reid Against the Theory of Ideas
by Bryan Register

Date: 1 May 97
Forum: University of Texas at Austin
Copyright: Bryan Register

Note: The author may or may not still agree with the views expressed in this paper.

'I cannot help thinking that the whole history of philosophy has never furnished an instance of an opinion so unanimously entertained by philosophers upon so slight grounds.' - Thomas Reid

The mainstream of early modern philosophers, including all of Descartes, Berkeley, and Hume, agree to the crucial thesis that the direct object of human awareness in perception is not an external object which exists independently of human consciousness, but rather an idea or impression. This paper will briefly review the recurrence of this theory, the 'Ideal Theory', in Descartes, Berkeley, and Hume, and attempt to explain their arguments for the thesis. Then I shall move on to Thomas Reid's opposition to this hypothesis and discuss his realism. I shall conclude with a rather startling result for the First Meditation from Reid's attack on the Ideal Theory.

In the Third Meditation, Descartes introduces the key distinction between objective and formal reality:

Moreover, even though the reality that I am considering in my ideas is merely objective reality, I ought not on that account to suspect that there is no need for the same reality to be formally in the causes of these ideas, but that it suffices for it to be in them objectively. For just as the objective mode of being belongs to ideas by their nature, so the formal mode of being belongs to the causes of ideas... (AT 42)

The distinction which he is trying to make is this: ideas have objective reality; they are the objects of thought. External objects, the causes of ideas, have formal reality; ideas also have formal reality inasmuch as they exist as modes of a mental substance in addition to existing as objects of awareness. The picture, then, is that we know ideas, which are either innate or caused by external objects. We do not contemplate the objects of ideas, we contemplate ideas.

Now, why does Descartes say this? In the Second Meditation, he explains the nature of mind: 'A thing that thinks. A thing that doubts, understands, affirms, denies, refuses, and also that imagines and senses' (AT 28). But there can be a mind even if there are no external things: 'For example, I now see a light, I hear a noise, I feel heat. These things are false, since I am asleep. Yet I certainly do seem to see, hear, and feel warmth. This cannot be false. Properly speaking, this is what in me is called 'sensing'. But this, precisely so taken, is nothing other than thinking' (AT 29). Thinking, then, does not imply the existence of an external object; the object of thought can - in fact, must - be internal to the thinking subject. Descartes' rationale for the objective-formal reality distinction would seem, then, to be his thesis of the separateness of mind and body, a view which he adopts because he can prove the existence of mind independently of body and form a clear and distinct idea of mind without any idea of body. So the cogito itself, with its conclusion of the existence of mind, is the root of the formal-objective reality distinction. (One might argue that these claims are consequences of, rather than premises for, the objective-formal reality distinction. This is true. However, this leaves the distinction totally groundless for Descartes, as he provides no clear argument for it. There is a question whether it is more charitable to Descartes to interpret him as making an unjustified claim, or to interpret him as arguing in circles, and I opt, for reasons of drama, for the second interpretation.)

Berkeley begins his Treatise of the Principles of Human Knowledge by claiming that 'It is evident to anyone who takes a survey of the objects of human knowledge, that they are... ideas actually imprinted on the senses...' (Principles, Part I, Section 1). At no later point in the Principles does Berkeley provide any argument for this view; Reid notes that 'The foundation on which such a fabric [as Berkeley's system] rests ought to be very solid and well-established; yet Berkeley says nothing more for it than that it is evident' (Essays On The Intellectual Powers, Essay II, ch. 10).

Hume, in section 12 of his Enquiry Concerning Human Understanding provides an argument for the view '...that nothing can ever be present to the mind but an image or perception...'(Enquiry, Section 12, Part 1) Hume argues that 'The table, which we see, seems to diminish, as we remove farther from it: But the real table, which exists independent of us, suffers no alteration: It was, therefore, nothing but its image, which was present to the mind' (ibid).

We shall now move on to Reid's rebuttal to Hume's argument and his general considerations against the Ideal Theory. Reid seeks to refute Hume's argument by bearing firmly in mind the very facts of perspective which make Hume's argument possible. For Reid,

...it is necessary to attend to a distinction which is familiar to those who are conversant in the mathematical sciences - I mean the distinction between real and apparent magnitude. The real magnitude of a line is measured by some known measure of length - as inches, feet, or miles.... Apparent magnitude is mesured by the angle which an object subtends to the eye (Essay II ch. 14).

That is, real magnitude is measured in units of length, while apparent magnitude is measured in degrees; the two are measures of different things: the one the amount of external space occupied by an object, the other the amount of one's visual field occupied by an object. Further, Reid wonders whether

it is... evident, that the apparent magnitude must continue the same while the body is unchanged? So far otherwise, that every man who knows anything of mathematics can easily demonstrate, that the same individual object, remaining in the same place, and unchanged, must necessarily vary in its apparent magnitude, according as the point from which it is seen is more or less distant... (ibid. emphasis added).

Reid's answer to Hume is that of course the object has a smaller apparent magnitude; the fact that we are directly aware of an external object, in combination with the laws of perspective, prove that the object must have a smaller apparent magnitude when our distance from it increases.

Reid could have pressed his attack with an interesting experiment. A foot-long object held six inches from the eye looks like a foot-long object held six inches from the eye, while a foot-long object on the other side of the room looks like a foot-long object on the other side of the room. We can be aware of this simply by consulting our own perceptual experience. Objects do not appear to shrink as they are distanced from an observer; they appear to be distanced from the observer.

(Since conventional visual arts attempt to present three-dimensionals objects on a two-dimensional surface, the artist, to accurately render perspective, ignores the perceived three-dimensionality and focuses exclusively on the amount of space and the exact shape of light reflected from an object as it falls on the retina. The viewer sees a painting as correctly rendering three-dimensional objects because his eyes do their normal job of seeing a flat field as in three dimensions depending on shading and perspectival effects.)

He might also have noticed that Hume's argument is circular. His first premise is that 'The table, which we see, seems to diminish as we remove farther from it...' But if the table-which-we-see seems to diminish, it has already been presupposed that the table-which-we-see is not the table in the external world.

Reid gives five additional arguments of his own against the Ideal Theory. He notes that the Ideal Theory 'is directly contrary to the universal sense of men who have not been instructed in philosophy' (ibid). This is a weak argument. It is still a common prejudice, almost 400 years after Galileo, that heavier things fall faster than light things; the 'universal sense of men' who have not been instructed in physics is false. That a position is counterintuitive is not an argument against it.

Second, he notes 'that the authors who have treated of ideas, have generally taken their existence for granted, as a thing that could not be called into question; and such arguments as they have mentioned incidentally, seem too weak to support the conclusion....' (ibid). This is a better point. It is the case that no argument appear in any of Descartes, Berkeley, or Hume until the last section of Hume's Enquiry, which argument is refuted above.

Third, he argues 'that philosophers, notwithstanding their unanimity as to the existence of ideas, hardly agree in any one thing else concerning them' (ibid). This too is a weak argument. Philosophers generally agree that the mind exists, but debate to no end about its nature. On Reid's argument, this should justify us in not accepting the thesis that the mind exists. Yet certainly Reid would not wish to deny the existence of the mind.

Fourth, he argues from parsimony 'that ideas do not make any of the operations of the mind to be better understood...' (ibid). This is quite right; in fact, the Ideal Theory leads to an untenable infinite regress. Hume argues from perceptual relativity to the conclusion that impressions are objects of awareness. But we can additionally argue from relativity that impressions have to be presented by some higher-level impression, to wit: qualitatively identical impressions take on a different appearance depending on the attitudes of the subject. There is no reason to suspect that this process, of an impression being presented by a higher-level impression, has a conclusion. On this infinitely regressive model of perception, no impression will ever actually be presented to the subject, and there will be no human experience, contrary to obvious fact.

Fifth and finally, Reid argues against the Ideal Theory

...that the natural and necessary consequences of it furnish a just prejudice against it to every man who pays a due respect to the common sense of mankind. Every man feels that perception gives him an invincible belief of the existence of that which he perceives... (ibid).

But this objection can be met by both Berkeley and Hume. For Berkeley, perception does give one an invincible belief of the existence of the perception's object - the object is simply that impression which is seen. Hume would agree with Reid that 'this belief is not the effect of reasoning' (ibid) but, since he mitigates his scepticism by accepting custom and habit as his guides, does not object to believing that there are external objects. He objects only to the notion that there is any reason to so believe.

Reid's attack on the Ideal Theory, then, is successful, though clouded by these several arguments from popular prejudice. When he argues from the facts of perspective, and the lack of an acceptable argument for the Ideal Theory, he is on solid ground; but much work must be done to ground his arguments from prejudice, if indeed they can be grounded at all.

I would like to conclude with an interesting point about the fate of Descartes' method of doubt. Descartes' method of doubt rests on the premise that it is possible to perceive something without it existing formally - or, colloquially, to bring out the point more clearly, that it is possible to see something without it being there. As stated, this is a contradiction. To coherently express the possibility of doubt, Descartes must say that it seems at least possible for there to be an entity with objective reality, which is unreflective of any external, formal reality. But such an expression rests on the distinction between objective and formal reality, which is not argued for anywhere in the Meditations. Descartes' method of doubt rests on the premise that the mind works in a certain way in perception, an unargued for premise. As a method of doubt, Descartes' sceptical arguments fail, because they all rests on a premise which is never called into question. The prime objection to Descartes should be, not that he is unable to get out of his doubts, but that he cannot get into them.

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